Low temperature restriction of charge shift in the primary reaction of the bacteriorhodopsin photocycle.

Fast electrical signals associated with the primary photoreaction of the bacteriorhodopsin photocycle were studied on dried oriented samples in the temperature range from 77 to 300 K. The rise of the electrical signal, associated with bathointermediate formation, was faster than 5 ns even at 77 K; no slow rising component was detected at any temperature in the nano- to microsecond time range. The amplitude of the signal associated with bathointermediate formation was not affected by cooling from 300 to 210 K, but decreased by a factor of two when the sample was further cooled from 210 to 190 K. At 77 K the amplitude from the first excitation flash is 25-30 per cent of that at 260 K. Our data suggest that low temperature restricts the size of the charge shift during the bathointermediate formation, resulting in creation of a "low temperature bathointermediate" distinct from the "room temperature bathointermediate".